67: Salsa r=matklad a=matklad
The aim of this PR is to transition from rather ad-hock FileData and ModuleMap caching strategy to something resembling a general-purpose red-green engine.
Ideally, we shouldn't recompute ModuleMap at all, unless the set of mod decls or files changes.
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>